1. Field of the Invention
The present invention relates to a recording medium, an optical pickup device, and a recording and reproducing apparatus, and more particularly to a recording medium having a plurality of recording layers for recording information by multi-photon absorption in a laminated direction, an optical pickup device compatible with the recording medium, and a recording and reproducing apparatus compatible with the recording medium.
2. Description of the Related Art
In recent years, as the capacity of a recording medium is increased, there has been developed an optical recording medium having a plurality of recording layers in a laminated direction. For instance, the optical recording medium may have an arrangement that the plurality of recording layers and one servo layer are laminated. In this recording medium, a flat recording layer without a guide track is formed, and a servo layer having a guide track is formed on the recording layer. In this arrangement, in performing a recording/reproducing operation, a focusing servo signal and a tracking servo signal are generated based on the guide track formed in the servo layer. Controlling the position of a beam spot on the recording layer based on the servo signals allows the beam spot to scan the recording layer along an intended scanning trajectory.
In the recording medium of the above kind, as the number of recording layers is increased, the distance between the servo layer and the recording layers in the laminated direction is increased. Accordingly, an excessively large number of recording layers may cause a problem that reliability of a servo signal with respect to recording layers is lowered. In view of this, the number of recording layers to be increased has a limit, and thereby the recording capacity of the recording medium is restricted.
The above problem may be solved by forming servo layers at a predetermined interval in the laminated direction, and reducing the number of recording layers to be formed between the adjacent servo layers. However, in this arrangement, laser light is less likely to reach a rear recording layer because the light amount attenuation resulting from a servo layer is increased.
Conventionally, there has been used a phase change material or a pigment material to perform a recording operation by absorbing one photon, as a recording layer material. In recent years, there has been proposed use of a so-called two-photon absorbing material having a property that the refractive index of a recording layer is changed by absorbing two photons, as a recording layer material. In addition to the above, it is possible to use a multi-photon absorbing material having a property that the refractive index of a recording layer is changed by absorbing three or more photons, as a recording layer material.
In the case where a two-photon absorbing material is used as a recording layer material, information is recorded by two-photon absorption. The two-photon absorption is a phenomenon that a material is excited by absorbing two photons. The probability of occurrence of two-photon absorption is proportional to the square of an incident light intensity (non-linear optical effect). In other words, two-photon absorption is induced solely in an area where the energy of incident light is concentrated. Condensing incident light through a lens enables to generate a condition that two-photon absorption occurs solely on a focal point including its vicinity, whereas two-photon absorption does not occur in the other space where light is not focused. Specifically, two-photon absorption can be induced in a two-photon absorbing material by condensing and irradiating laser light having an absorption wavelength bandwidth of the two-photon absorbing material with an ultra-short pulse and a high intensity. There is also proposed a method for inducing two-photon absorption in a two-photon absorbing material by condensing and irradiating laser light having an absorption wavelength bandwidth of the two-photon absorbing material, while irradiating light of a wavelength other than the absorption wavelength bandwidth. In the latter method, the energy level of the two-photon absorbing material is raised by the light of the wavelength other than the absorption wavelength bandwidth. Accordingly, two-photon absorption is induced, even if the intensity level of laser light having the absorption wavelength bandwidth is suppressed to some extent.
In the case where the material of the above kind is used in a recording layer of the multilayer recording medium, it is necessary to provide an arrangement for smoothly and efficiently guiding laser light to a rear recording layer in performing recording and reproducing operations. In particular, since it is necessary to irradiate laser light of an ultra-short pulse and a high intensity on a recording layer in performing a recording operation, it is necessary to cause the laser light to reach the rear recording layer while suppressing attenuation.